Conventionally, there has been provided an image forming apparatus, such as a digital copying machine, which reads an original document placed on an original document table or platen, formed of a transparent material such as glass, by exposing the document through the transparent original document table and scanning the exposed document with a scanning means, such as a scanner, so as to form an image.
In such an apparatus, an original document positioning member such as a scale is generally provided at an end portion of the original document table, where the document positioning member abuts against an end portion of an original document so as to position the original document in a specified placement position. This kind of original document positioning member includes a fixed type, which is provided in close contact with the end portion of the original document table, and a movable type, which can be located at a desired distance from the end portion of the original document table.
Among these, the movable type of original document positioning member is frequently adopted, particularly in an image forming apparatus which is provided with an automatic document feeder. The movable type of document positioning member is provided adjacent to the end portion of the original document table which is on the downstream side, in an original document feeding direction of the automatic document feeder, and operates to position the original document by abutting against the leading end of the original document by protruding a specified height above an upper surface of the original document table while the original document is being fed onto the original document table by the automatic document feeder and by retracting into a position lower than the upper surface of the original document table while the automatic document feeder discharges the original document from the original document table after scanning the original document, so as not to hinder the operation.
However, the apparatus provided with the movable type of original document positioning member has had problems as follows. In order to make the movable type of document positioning member movable between a first position in which the document positioning member protrudes above the upper surface of the original document table and a second position in which the document positioning member is lowered below the upper surface of the original document table, depending on the state of operation of the automatic document feeder, a specified gap must be provided between the document positioning member and the adjacent end portion of the original document table.
Therefore, when the original document is exposed and scanned by the scanning member, such as a scanner, in a state in which the original document has been placed so as to abut against the document positioning member, there is a possibility that the original document may be directly scanned by scanning light which has passed through the space between the document positioning member and the adjacent end of the original document table, and a possibility that the scanning light is interrupted by an extreme end portion of the original document table, possibly causing defective reading and loss of image.
However, these problems do not cause any particular disadvantage in most procedures for normal image formation wherein an image is to be formed on a paper sheet which is of the same size as the original document. This is because such procedure intentionally avoids the formation of any image of a specified extent of the peripheral edge portions of the original document, in order to prevent the possible occurrence of a jam at a fixing section, to reduce the smear of the apparatus body by the toner, and for other reasons.
However, as original documents having an image on the full extent of the document (e.g., photographs) have become more common in accordance with the recent popularization of full-color copying machines, there is a growing demand for forming an image of the entire extent of the original document. In such a case, the defective reading and the loss of image at the end portion of the original document, due to the movable type of document positioning member as described above, become serious problems.